1. Field of the Invention
The present invention relates to an optical system for use in an image projector (hereafter such an optical system will be referred to as a "projector optical system"), and more particularly to a projector optical system for use in an image projection apparatus (such as a liquid crystal projector) for projecting an image from a reflection-type display panel (such as a reflection-type liquid crystal panel) onto a screen.
2. Description of the Prior Art
In recent years, reflection-type LCD (liquid crystal display) panels are receiving much attention. This is because, as compared with a transmission-type LCD panel, a reflection-type LCD panel offers a higher aperture ratio and thus allows more efficient use of light. Illumination light shone onto the display surface of such a reflection-type LCD panel is reflected therefrom as projection light that has a reflection angle substantially of the same magnitude as but of the opposite sign to the incident angle of the illumination light. Thus, by illuminating the display surface of a reflection-type LCD panel substantially perpendicularly, it is possible to project an image by using the projection light exiting therefrom substantially perpendicularly. FIG. 13 shows an example of a projector optical system that projects an image in this way.
As shown in FIG. 13, the illumination light emitted from a light source (1) is first reflected from a reflector (2), and then passes through an illumination optical system (OP1). The illumination optical system (OP1) includes an integrator, a polarization conversion optical system, and other components. The integrator serves to illuminate reflection-type LCD panels (pr, pg, and pb) evenly and efficiently. The polarization conversion optical system makes the polarization direction of the illumination light uniform. After passing through the illumination optical system (OP1), the illumination light, now polarized on a particular polarization plane, is separated into three light components of different colors R (red), G (green), and B (blue) by a color separating optical system composed of two dichroic mirrors (M1 and M2). Then, the B light component of the illumination light enters a polarizing beam splitter (BS2); the R light component of the illumination light enters a polarizing beam splitter (BS2); and the G light component of the illumination light enters a polarizing beam splitter (BS3). These polarizing beam splitters (BS1 to BS3) are disposed with their S-polarization plane aligned with the polarization plane of the illumination light. Thus, only S-polarized light included in the illumination light is reflected from the polarizing beam splitters (BS1 to BS3) so that the individual light components of different colors enter the display surfaces of the corresponding reflection-type LCD panels (pr, pg, and pb) perpendicularly.
The illumination light incident on the reflection-type LCD panels (pr, pg, and pb) is reflected therefrom as partially P-polarized and partially S-polarized light according to the pattern formed by the pixels of the LCD panels. Then, the projection light (still including P-polarized and S-polarized light) regularly reflected perpendicularly from the reflection-type LCD panels (pr, pg, and pb) enters the polarizing beam splitters (BS1 to BS3) once again. Since the polarizing beam splitters (BS1 to BS3) reflect S-polarized light and transmit P-polarized light, only that portion of the illumination light which was converted into P-polarized light by the reflection-type LCD panels (pr, pg, and pb) is, as projection light, allowed to enter a projection lens system (OP2). The light components of three colors constituting the projection light (now including only P-polarized light) transmitted through the polarizing beam splitters (BS1 to BS3) are integrated together by a color integrating optical system composed of a cross dichroic prism (DP), and is then projected through the projection lens system (OP2) to form an image on a screen (not shown).
In the above-described conventional arrangement (see FIG. 13), the polarizing beam splitters (BS1 to BS3) determine, by transmitting only P-polarized light, whether the individual pixels are "on" or "off". Thus, the characteristics of the polarizing beam splitters (BS1 to BS3) (i.e. how far they can cut the portion of the projection light which strikes the "off" pixels) affect the contrast of the projected image. However, since the characteristics of the polarizing beam splitters (BS1 to BS3) tend to vary with the internal strain of its glass material and with the incident angle of light rays, the use of polarizing beam splitters (BS1 to BS3) does not necessarily ensure high performance.
To avoid such a problem, various projector optical systems have conventionally been proposed that employ no polarizing beam splitter. FIGS. 14 and 15 show an example of such a projector optical system. In FIGS. 14 and 15, X, Y, and Z represent directions perpendicular to one another, with the direction of the optical axis of the projection lens system (OP2) used as the Z direction. The section (the YZ section) of he optical path along which an illumination optical system (OP1) achieves illumination is substantially perpendicular to the section (the XZ section) of the optical path along which a cross dichroic prism (DP) performs color separation and integration. Both the illumination principal ray (L1) (the light ray traveling from the beam center of the illumination light emitted from the illumination optical system (OPI) to the center of a reflection-type LCD panel (PG)) and the projection principal ray (L2) (the light ray traveling from the center of the reflection-type LCD panel (PG) to the center of the aperture stop of the projection lens system (OP2)) lie on the YZ plane.
The illumination light emitted from a light source (1) is first reflected from a reflector (2), and then passes through an illumination optical system (OP1). The illumination optical system (OP1) includes, like that of the conventional example shown in FIG. 13, an integrator, a polarization conversion optical system, and others. The illumination light has its polarization direction aligned with the Y' direction (the direction perpendicular to the illumination principal ray (L1) and lying on the YZ plane) by the illumination optical system (OP1), is then subjected to color separation by a color separating/integrating optical system composed of a cross dichroic prism (DP), and then enters a blaze-formed diffraction grating (GP) (not shown in FIG. 15). The blaze-formed diffraction grating (GP) is disposed immediately in front of each of reflection-type LCD panels (PR, PG, and PB) so as to serve as an auxiliary optical system that allows the illumination light obliquely incident on the individual reflection-type LCD panels (PR, PG, and PB) to be reflected therefrom substantially perpendicularly.
The illumination light transmitted through the blaze-formed diffraction grating (GP) then enters the reflection-type LCD panels (PR, PG, and PB). Each reflection-type LCD panel (PR, PG, and PB) has a polarizing plate (PP) disposed on the cross-dichroic-prism (DP) side thereof. The polarizing plate (PP) is disposed with its polarization axis (AXP) (the polarization direction) aligned with the Y direction. Thus, of the incident light transmitted through the polarizing plate (PP), that portion which has its polarizing plane rotated by being reflected from the reflection-type LCD panels (PR, PG, and PB) is, when exiting, cut by the polarizing plate (PP). The projection light exiting from the reflection-type LCD panels (PR, PG, and PB) is, after passing through the blaze-formed diffraction grating (GP), integrated together by the cross dichroic prism (DP) and then projected through the projection lens system (OP2) to form an image on a screen (not shown).
Note that projector optical systems having the same structure as shown in FIGS. 14 and 15 are proposed by Japanese Laid-Open Patent Application No. S63-292892 and others. However, these projector optical systems do not include a polarization conversion optical system within their illumination optical system. A reflection-type LCD panel is designed to accept only light that is polarized on a particular polarizing plane by being transmitted through a polarizing plate. Accordingly, the portion of the illumination light that is not polarized in a specific direction by a polarization conversion optical system is not allowed to pass through the polarizing plate, and thus is not used. This makes efficient use of light impossible.
As shown in FIGS. 14 and 15, by using an illumination optical system (OP1) that includes a polarization conversion optical system, the above-described problem can be prevented. In this case, however, the following problem arises instead. As shown in FIG. 15, a polarizing plate, which is cemented on the surface of a reflection-type LCD panel, is typically disposed with its polarization direction aligned either with the short-side direction of the panel (in FIG. 15, the Y direction) or with the long-side direction of the panel (in FIG. 15, the X direction). Thus, the illumination light emitted from the illumination optical system (including a polarization conversion optical system) is set to travel with its polarization plane aligned either with the YZ plane shown in FIG. 15 or with the X direction perpendicular to the YZ plane.
In the conventional example shown in FIG. 15, the polarization direction (AXP) of the polarizing plate (PP) is aligned with the Y direction, and the polarization direction of the illumination light emitted from the illumination optical system (OP1) is aligned with the Y' direction. Thus, if no consideration is given to the dichroic prism (DP), the illumination light having a polarization plane parallel to the YZ plane is allowed to pass through the polarizing plate (PP). In this case, however, due to the positional relationship between the optical path from the illumination optical system (OP1) and the dichroic surface of the cross dichroic prism (DP), the illumination light that is supposed to be polarized wholly in the Y' direction is, when passing through the dichroic surface, separated into a light component polarized in the X direction and a light component polarized in the Y' direction.
This occurs because the polarized light is deviated from the S- and P-polarization planes when entering the dichroic surface. If a deviation in the phase or the like occurs additionally, the light will be polarized elliptically. The degree of the above-mentioned separation is wavelength-dependent. For example, FIG. 16 shows the spectral transmittance of the light components polarized in the X and Y' directions (indicated by the broken and solid lines, respectively) as observed when the illumination light is shone onto the dichroic surface that reflects the B light component. The reflection-type 1, CD panels (PR, PG, and PB) are not designed to transmit a light component polarized in the X direction. Therefore, if the polarized light includes a light component polarized in the X direction, it is impossible to achieve efficient use of light. Moreover, the characteristics related to spectral transmittance vary with the incident angle of the illumination light. As a result, it is impossible to achieve even color distribution.